I've thought for awhile that a number of Pokémon abilities seem to be conducive to causing various transformations to the like. That thinking also led to the idea that the ability Trace, which causes Pokémon with it to take on whatever ability their opponent has, could result in some surprise transformations because of this.
To that end, I created a Gardevoir OC named Violet, who basically has a Trace ability that is constantly giving her abilities from various passing Pokémon, while also causing her body to transform in lewd ways to match whatever ability she gets. So, for example, if she Traces Goodra's Gooey ability, she transforms into a goo girl. Bear in mind that breast expansion and lactation are common themes in most of these.
Without further over explanation, you can find an imgur gallery with all of the pictures I've commissioned of Violet and her various transformations thus far here.
For those who want to see this kind of idea done with someone other than me behind it,
this artist on DA was doing pics of Gardevoir transforming to fit the abilities she Traces before I did; however, his transformations were more for comedy and less for lewd purposes than mine.